Mobile access to data services and speech services requires fast and reliable wireless communication channels. Frequency-Division Multiple-Access (FDMA) is an example of a widely implemented multiple access scheme, including Orthogonal FDMA (OFDMA) in a downlink channel and Single Carrier FDMA (sc-FDMA) in an uplink channel of 3GPP Long Term Evolution (LTE). FDMA multiplexes the transmitted data on a plurality of subcarriers in the frequency dimension. FDMA thus provides a high data throughput rate and robustness against multipath interference due to frequency diversity. In addition, symbols can be stretched in time so as to exceed a delay spread of the wireless communication channel. Moreover, antenna diversity or spatial multiplexing can be combined with the frequency diversity and the temporarily stretched symbols to further increase robustness and data rates. Hence, frequency, time and space are examples for the transmit dimensions of a communication channel.
Harnessing the advantages of such modern transmission techniques often requires detailed knowledge of the state of the wireless communication channel in each combination of its different transmit dimensions. To determine the channel state at the receiver, a plurality of Reference Signals (also referred to as pilot signals) are assigned both in the time-domain and in the frequency-domain to a Resource Block prior to transmission. FIGS. 2(a) and (b) of research article “Channel Estimation for OFDMA systems based on Comb-type Pilot Arrangement in Frequency Selective Fading Channels” by M.-H. Hsieh et al., IEEE Transactions on Consumer Electronics, Vol. 44, Issue 1, pages 217-225, show exemplary subcarrier arrangements of the Reference Signals.
Document US 2011/0216842 A1 describes a wireless communication channel between a base station and a mobile device (also referred to as User Equipment, UE). The base station uses predetermined subcarriers for UE-specific Reference Signals.
LTE standard document 3GPP TS 36.213 (Release 11, Version 11.2.0) defines Transmission Modes with and without spatial multiplexing. Precoding for spatial multiplexing is described in Sect. 6.3.4.4 of 3GPP TS 36.211 (Release 11, Version 11.1.0). Transmission Mode 7 uses a single antenna port with UE-specific Reference Signals. Positions of the Reference Signals in time and frequency are defined in Sects. 6.2.1 and 6.10 of LTE standard document 3GPP TS 36.211 (Release 11, Version 11.1.0).
The Reference Signals are used for channel estimation at the given position in time and frequency. In principle, it is possible to apply filters to the Reference Signals providing channel estimates for each of the required Resource Elements within the Resource Block. Seminar report “Reference Signals and Channel Estimation” by L. D. C. Anthony, Seminar on Selected Chapters of Telecommunications, describes filters for channel estimation in the frequency direction (Sect. 4.2), in the time direction (Sect. 4.3) and in the spatial direction (Sect. 4.4). Such filters obey a specialized filter criterion, such as Minimum Mean Square Error (MMSE) or Least Square (LS). For example, a one-tap LS estimation can be applied to the Reference Signals for a coarse channel estimation that is input to an MMSE estimation resulting in channel estimates for each of the Resource Elements in the Resource Block, as is described in Sect. IV of research publication “Robust MMSE Channel Estimation in OFDM Systems with Practical Timing Synchronization” by V. Srivastava et al., WCNC 2004, pages 711-716.
However, the filtering is computationally expensive. Therefore, the channel estimates are computed by means of computationally complex filters only for a subset of non-Reference Signal positions. For the remaining Resource Elements, the channel estimates are linearly interpolated based on the filtered channel estimates.
While such a simplification of channel estimation describes the channel state in frequency, time and/or space with sufficient accuracy for some situations, there remain situations for which the simplification leads to poor channel estimates reducing the advantages of modern transmission techniques such as FDMA.